Leukemias encompass hematological malignancies, characterized by the clonal expansion of hematopoietic cells exhibiting abnormal proliferation, blocked differentiation, and reduced apoptosis. Leukemias are generally categorized into multiples types, such as acute or chronic, or myeloid or lymphoid, depending on the rate of disease progression and hematopoietic lineage, with further classification into subtypes based on the stage of differentiation blockage.
Chronic myelogenous leukemia (CML) is also known as chronic myeloid leukemia, chronic myelocytic leukemia, or chronic granulocytic leukemia. The majority of patients diagnosed with CML have a characteristic chromosomal abnormality called the Philadelphia chromosome which results in the formation of the BCR-ABL fusion protein. CML generally progresses through three phases characterized by the number of immature leukemic cells in the blood and bone marrow of the patient. Traditional therapies for CML include chemotherapy, bone marrow transplantation, and interferon therapy, although targeted therapies are under development.
Acute myelogenous leukemia (AML), also known as acute myeloid leukemia, acute myelocytic leukemia, acute granulocytic leukemia, and acute non-lymphocytic leukemia, is characterized by abnormal proliferation of myeloid cells. The development of AML is thought to be mediated by two major mechanisms: increased self-renewal and a block in differentiation of the leukemia cells (Gilliland et al. 2004), in contrast to differentiation in normal hematopoiesis, which is mediated by lineage specific transcription factors (e.g. CEBPα, PU-1) directing the hematopoietic stem cells to differentiate into progenitor cells and finally to mature blood cells (Koschmieder et al. 2005). AML blasts, reflecting the immature, accumulating leukemia cells, can display a block in differentiation at any maturation level.
Standard treatment of AML to date includes intensive chemotherapy and bone marrow transplantation (Hiddemann et al. 2005). The overall long-term survival rate for AML patients is between 20-30%, depending on the treatment regime and study (Buechner et al. 2005). As chemotherapy is not a targeted therapy, AML patients often suffer from side-effects and relapse of the disease. The current standard chemotherapy for AML is a 30 year old treatment regime with cytarabine and an anthracycline, which is cytotoxic and has other deleterious side effects. Despite emerging novel targeted therapies like FLT3 inhibitors and anti-CD33 antibodies, only the introduction of all-trans-retinoic-acid (ATRA) for acute promyelocytic leukemia (also known as APL or AML M3) has provided a differentiating treatment for leukemia cells. The use of ATRA combined with chemotherapy has increased long term survival to 80-90% in APL patients (Kuchenbauer et al. 2005). However, many AML patients are resistant to exogenous differentiating agents, including ATRA (Hiddemann et al. 2004).
Micro RNAs (miRNAs) are generally 21-24 nucleotide (nt) long RNA molecules and are thought to be important posttranscriptional regulators of mRNAs (Bartel 2004). mRNAs are initially transcribed as longer RNA molecules called primary-miRNAs (pri-miRNAs), but undergo a multistep maturation process involving cleavage through Drosha (nuclear) and Dicer (cytoplasmic) (Kim 2005). The resulting double stranded RNA molecule, consisting of the mature miRNA strand and its partially complementary strand counterpart miRNA star (miRNA*), enters a protein complex named RNA induced silencing complex (RISC) that uses the strand with the mature miRNA sequence as template for degradation of the specific, complementary mRNA (Kim 2005). The mature miRNA is characterized by a “seed region”, generally comprising the bases 2-7 of the 5′ end (Lewis et al. 2005). The seed region is thought to primarily define the specificity of a miRNA towards the 3′UTR of its target mRNAs and has been used for computational target predictions (Lewis et al. 2005). For each miRNA a few hundred target mRNAs are predicted, whereas relatively few targets have been experimentally validated to date. Recent deep sequencing approaches led to changes in the current miRNA databases and implicate miRNA* as an active miRNA molecule (Ruby et al. 2006). Furthermore, in some miRNA stemloops, such as mir-302b, both the 5′ and the 3′ stemloop sequences are annotated as mature miRNAs, suggesting that both miRNA strands can have functional properties.
In general, miRNAs are closely related to siRNAs, which are double-stranded RNA molecules that are also processed by Dicer. In contrast to primitive organisms like C. elegans, endogenous siRNAs rarely occur in mammals and consequently do not play a physiological role (Aravin and Tuschl 2005).
A connection between miRNAs and hematopoiesis was made when it was demonstrated that certain miRNAs are expressed within specific hematopoietic lineages and that their expression levels regulate hematopoietic differentiation (Chen et al. 2004). MicroRNA profiling in chronic lymphatic leukaemia and other lymphomas suggests that miRNA expression changes during pathogenesis and implied that miRNAs may play a functional role in hematopoietic disorders. Overexpression of miR-155 and the miR-17-19b cluster was implicated in promoting the development of lymphomas, suggesting that miRNAs can act as oncogenes (Costinean et al. 2006; He et al. 2005).
Relatively little is known about the role of miRNAs in leukemias of myeloid origin. A study correlating the expression levels of 5 miRNAs with the genome-wide mRNA expression profiles of the same leukemias suggested that miR-181a correlates strongly with the AML morphological sub-type and with the expression of genes previously identified through sequence analysis as potential interaction targets (Debernadi et al. 2007). Another study suggested that expression of miR-223 induces granulocytic differentiation in an acute promyelocyte leukemia (APL) model and is controlled by a regulatory circuitry involving miR-223 and two transcriptional factors, NFI-A and CEBPalpha (Fazi et al. 2005). It has been also reported that miR-223 expression underlies a highly conserved transcriptional mechanism involving the myeloid transcription factors PU-1 and CEBPalpha (Fukao et al. 2007).